in my opinion (and thrust me, i'm a laymen in physics no more then you are, more of a chemistry guy
) the box wont hold because of continous process. The function of the fin can be reduced to a mere point of plane departure whereas the sail is the driving power. In speed now we take fins into much more account: a good fin = good final speed. However, should we lower the point of effort of the sail to the harneshook like they have in kitesurfing (all pull of kite goes to you body through 1 point of effort whereas a sail will still pull on your arms in different angles etc) we could handle bigger sails and thus achieve higher speed without having to adapt our fins. They'd just be a planing departure point of the board .... That means that 'the box' of good sail + good board + good adapted fin no longer holds that well and you ascend from the box.
my recon ... but i may be way off